1. Field of the Invention
The present invention relates to homodyne communication systems and in particular to homodyne vehicle location systems.
2. Description of the Prior Art
Communication systems utilizing interrogation units and responsive identification tags which provide a modulated return signal to the interrogating unit are described in copending applications Ser. No. 428,721, filed Dec. 27, 1973 by R. J. Klensch and Ser. No. 473,765, filed May 28, 1974 by F. Sterzer, cited above. Such systems however, utilize pulse (amplitude) modulation of the return signal and direct detection of the return signal modulation in the interrogating unit. Consequently, such systems are disadvantageous with respect to, for example, a homodyne system, in that direct detection is not as sensitive as a homodyne system at low power signal levels, and thus, requires higher power signal transmission levels of the interrogation signals. Moreover, direct detection is not compatible with phase modulation, which is advantageous with respect to amplitude modulation in that better signal to noise ratios may be achieved with phase modulation.
A homodyne system is defined in the art as a system wherein a sample of the transmitted signal is used as a local oscillator signal. Typically, in a homodyne system, an RF signal is transmitted from an interrogating unit to illuminate a compatible remote unit. The remote unit casues a signal to be transmitted or reflected back to the interrogating unit. A sample of the transmitted (interrogation) signal is applied, with the returned (reply) signal to a suitable phase detector or mixer. In general, the amplitude of the output signal of the mixer is equal to the product of the amplitudes of the sample of the transmitted signal (local oscillator signal), and the returned signal, and the cosine of the relative phase angle.
It should be noted, that since the interrogation and reply signals are derived from the same source, the relative phase of such signals is a function of the round trip distance between the interrogating unit and the remote unit. Such a phenomenon is the basis of many radar ranging systems, which compute the distance between interrogating and remote units from the relative phase of the signals. However, at various phase relationships, e.g. when the sample of the interrogation signal and reply signal are 90.degree. out of phase, there is a null in the amplitude of the mixer output signal.
In a homodyne communication system, wherein the reply signal is modulated with signals bearing information, such amplitude nulls may cause serious errors in processing the information, such errors being termed herein as amplitude null errors. Specifically, in systems wherein the interrogating unit may be in relative motion with respect to the remote unit, various relative positions of the interrogating and remote units give rise to phase relationships causing such amplitude null errors, thereby introducing potential errors in or loss of the communicated information.
It will be appreciated that a drift in the frequency of the interrogation signal will cause the amplitude null producing relative positions to change, giving thereby the effect of a relative movement of the units.